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Current time:0:00Total duration:4:12

- [Voiceover] If H-A is a
generic acid and donates this proton, the electrons in
this bond, so the electrons in magenta are left behind
on A to form A minus. So A minus is the conjugate base to H-A. If A minus is stable, then H-A is more likely to donate this proton. Therefore, if you want
to determine the acidity of a compound, you can
look at the stability of the conjugate base. The more stable the conjugate
base, the more likely the acid is to donate a proton. Therefore, the more
stable the conjugate base, the stronger the acid. Let's use that concept and let's look at these four compounds down here. So we'll start with methane. The PKA for this proton on
methane is approximately 48. For ammonia the PKA for
this proton is about 36. And if we look at water, the PKA for this proton is about 16 and finally for H-F, this proton has a PKA of about three. We know that the lower the PKA
value, the stronger the acid so as we move to the
right we see a decrease in PKA values from 48 to 36 to 16 to three. Therefore, as we go to the right we see an increase in acid strength. We see an increase in acid strength so H-F is the strongest acid out of these four. And if H-F is the strongest
acid out of these four, then H-F must have the
most stable conjugate base. So now let's think about
the conjugate bases for all four of these compounds
so let me go down here and we'll get some more room. If we take this proton
from methane, then these electrons are left behind on the carbons. So the carbon gets a
negative one formal charge. For ammonia if we took this
proton, then these electrons are left on the nitrogen
so the nitrogen has a negative one formal charge. For water, if we took this
proton these electrons are left on the oxygen to form
the hydroxide anion as our conjugate base with a negative one formal charge on the oxygen. And finally if we took this
proton, then these electrons would be left behind on the fluorine to form the fluoride anion. We already know that H-F
is the strongest acid out of these four and the
strongest acid must have the most stable conjugate base. So the fluoride anion must be
the most stable conjugate base so as we move to the right we're increasing in the stability. We're increasing in the
stability of the conjugate base and we can explain that
trend by looking at the element that has the
negative charge, right? This has a negative charge on carbon and this was our least stable conjugate base. Then we go to nitrogen
with a negative charge. We get a little bit more stable. We go to oxygen with
a negative one charge. We get a little bit more stable. And finally we get to fluorine
with a negative charge and we had the most stable conjugate base. That's the same trend as
electronegativity so if you look at carbon, nitrogen, oxygen,
and fluorine as you move to the right on the periodic
table you know you increase in electronegativity with fluorine being the most electronegative
element and the most electronegative element
attracts electrons the most. It likes to have electrons around it and therefore it makes sense
that fluorine is the best at stabilizing a negative
charge and that makes this the most stable conjugate base. And if this is the most
stable conjugate base, the fluoride anion is the
most stable conjugate base, that means that H-F must
be the strongest acid. So when you're thinking
about acid strength, think about the stabilization
of the conjugate base and the different
factors that can do that. Electronegativity is one of
those factors to think about.